(1) Field of the Invention
The present invention relates to tip devices for aerodynamic lifting surfaces. In principle devices according to the invention may be employed to improve the performance of lifting surfaces of numerous kinds including aircraft wings, helicopter blades, propeller blades and so on, although they may find particular utility as extensions for the main wings of high subsonic speed, medium and long range passenger and transport aircraft, including airliners and corporate/business jets, of the class containing examples to which conventional winglets are currently fitted.
(2) Description of the Art
There is always a desire to improve the performance of aircraft by reducing drag. For high subsonic speed aircraft the three main drag components are viscous, wave and lift-dependant (i.e. induced) drag and for a typical high speed cruise condition approximately 50% of the total drag force is due to induced drag. A strong interest therefore exists in the effects of wing planform and tip shape on induced drag. One result has been the fitting of so-called winglets to such aircraft as the Airbus A340 and Boeing 747-400. These winglets are upwardly canted, generally planar devices extending from the wing tips which effectively increase the aspect ratio of the wing and utilise the cross flow occurring around the tip to reduce the intensity and concentration of the trailing vortex and thereby reduce the induced drag. A developed form of such devices referred to as a “blended winglet” is described in U.S. Pat. No. 5,348,253. Another form of upwardly canted wing tip extension is described in U.S. Pat. No. 6,722,615. U.S. Pat. Nos. 5,039,032 and 6,089,502 describe another approach comprising highly tapered and raked back but uncanted wing tips, such as are fitted to the Boeing 767-400ER aircraft.
It is evident from a study of wing tip shapes that the most effective from the point of view of drag reduction are those which (i) improve the wing span-loading, (ii) position the tip vortex further outboard, and/or (iii) introduce a non-planar trailing vortex sheet, since one or a combination of these effects can result in a beneficial change to the downwash and hence a reduction in the induced drag. However, all such devices as described above carry the penalties of adding their own weight to the structure, additional drag due to increased wetted area, adverse interference and wing bending. One of the main concerns is the increase in the wing root bending moment in flight, due to the additional lift which they produce and its siting at the maximum span position. A consequence of  meeting an increased wing root bending moment is an increase in the required wing structural strength and hence a still further increase in weight.